Facet joint replacement

ABSTRACT

A prosthesis for the replacement of a diseased or traumatized facet of a mammalian vertebra includes a surface that articulates with another prosthetic facet or a natural facet, a portion that replaces at least a bony portion of the diseased or traumatized spine facet which is to be replaced, and an element to attach the prosthesis to the vertebra in a manner that does not require attachment to or abutment against the posterior arch. A method of installing the prosthesis includes the steps of resecting at least a portion of a facet and attaching the prosthesis in a manner that does not require attachment or abutment against the posterior arch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of:

-   -   U.S. application Ser. No. 10/421,078, filed Apr. 23, 2003 now        U.S. Pat. No. 7,041,136 and entitled FACET JOINT REPLACEMENT,

Which is a continuation of:

-   -   U.S. application Ser. No. 09/726,169, filed Nov. 29, 2000 and        entitled FACET JOINT REPLACEMENT, which is now issued as U.S.        Pat. No. 6,579,319.

The disclosures listed above are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to surgical devices and methods to replacea damaged, diseased, or otherwise painful spinal facet joint.

2. The Relevant Technology

Traumatic, inflammatory, metabolic, synovial, neoplastic, anddegenerative disorders of the spine can produce debilitating pain thatcan have severe socioeconomic and psychological effects. One of the mostcommon surgical interventions today is arthrodesis, or spine fusion, ofone or more motion segments, with approximately 300,000 proceduresperformed annually in the United States. Clinical success variesconsiderably, depending upon technique and indications, andconsideration must be given to the concomitant risks and complications.For example, it has been shown that spine fusion decreases function bylimiting the range of motion for patients in flexion, extension,rotation, and lateral bending. Furthermore, it is believed that spinefusion creates increased stresses and, therefore, accelerateddegeneration of adjacent non-fused motion segments. Additionally,pseudoarthrosis, as a result of an incomplete or ineffective fusion, mayreduce or even eliminate pain relief for the patient. Finally, thefusion device, whether artificial or biological, may migrate out of thefusion site.

Recently, several attempts have been made to recreate the naturalbiomechanics of the spine by use of an artificial disc. Artificial discsprovide for articulation between vertebral bodies to recreate the fullrange of motion allowed by the elastic properties of the naturalintervertebral disc which directly connects two opposed vertebralbodies.

However, the artificial discs proposed to date do not fully address themechanics of motion of the spinal column. In addition to theintervertebral disc, posterior elements called the facet joints help tosupport axial, torsional and shear loads that act on the spinal column.Furthermore, the facet joints are diarthroidal joints that provide bothsliding articulation and load transmission features. The effects oftheir absence as a result of facetectomy is believed to producesignificant decreases in the stiffness of the spinal column in allplanes of motion: flexion and extension, lateral bending, and rotation.Furthermore, contraindications for artificial discs include arthriticfacet joints, absent facet joints, severe facet joint tropism orotherwise deformed facet joints.

U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial facet jointwhere the inferior facet, the mating superior facet, or both, arecovered with a cap. The cap requires no preparation of the bone orarticular surfaces; it covers and, therefore, preserves the bony andarticular structure.

The capping of the facet has several potential disadvantages. If thefacet joint is osteoarthritic, a cap will not remove the source of thepain. Additionally, at least in the case of surface replacements forosteoarthritis femoral heads, the capping of articular bone ends hasproven to lead to clinical failure by means of mechanical loosening. Theclinical failure is hypothesized to be a sequela of disrupting theperiosteum and ligamentum teres femoris, both serving a nutritiondelivery role to the femoral head, thereby leading to avascular necrosisof the bony support structure for the surface replacement. Anotherpotential disadvantage is that in order to accommodate the widevariability in anatomical morphology of the facets, not only betweenindividuals but also between levels within the spinal column, a verywide variety of sizes and shapes would be required.

U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet jointprosthesis that is supported on the lamina, or the posterior arch of thevertebra. Extending from this support structure are inferior and/orsuperior blades that replace the cartilage at the facet joint. Like theFitz design, the Martin prosthesis generally preserves existing bonystructures and therefore does not address pathologies which affect thebone of the facets in addition to affecting the associated cartilage.Furthermore, the Martin invention requires a mating condition betweenthe prosthesis and the lamina, or the posterior arch, that is a thinbase of curved bone that carries all four facets and the spinousprocess. Since the posterior arch is a very complex and highly variableanatomic surface, it would be very difficult to design a prosthesis thatprovides reproducible positioning to correctly locate thecartilage-replacing blades for the facet joints.

Another approach to surgical intervention for spinal facets is providedin WO9848717A1 to Villaret. While Villaret teaches the replacement ofspine facets, the replacement is interlocked in a manner to immobilizethe joint.

Facet joint replacement in conjunction with artificial disc replacementsrepresent a unique solution to recreating a fully functional motionsegment that is compromised due to disease or trauma. Together, facetjoint and disc replacement can eliminate all sources of pain, returnfull function and range of motion, and completely restore the naturalbiomechanics of the spinal column. Additionally, degenerative ortraumatized facet joints may be replaced in the absence of discreplacement when the natural intervertebral disc is unaffected by thedisease or trauma.

It would therefore be an improvement in the art to provide a vertebralfacet replacement device and method that replaces a bony portion of thefacets so as to remove the source of arthritic, traumatic, or otherdisease mediated pain.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an artificial vertebralfacet that replaces the cartilage and a portion of the bone of a facet.

It is a further object of the invention to provide a method forpreparing a facet for the installation of an artificial vertebral facet.

It is another object to provide a method for replacing a spinal facet.

It is yet another object of the invention to provide a total vertebralfacet joint replacement.

In the preferred embodiment, an inferior facet of a superior vertebra isresected at the base of the facet where it connects to the posteriorarch. The fin of a prosthetic inferior facet is pressed into theinterior bone space of the posterior arch. Alternatively, a tool, suchas a broach or punch, may be used to first prepare a space for the finwithin the posterior arch.

Alternatively, or in addition, a superior facet of an inferior vertebrathat articulates with the inferior facet is resected at the base of thefacet where it connects to the pedicle. The post of a prostheticsuperior facet is pressed into the interior bone space of the pedicle.Alternatively, a tool, such as a broach or punch, may be used to firstprepare a space for the post within the pedicle.

The post and the fin may be porous coated to promote bone ingrowth inorder to achieve long term fixation. Acute fixation is provided by apress fit between the post or fin and the internal surface of the bone.The porous coating may carry osteoconductive agents, such ashydroxylapatite, calcium sulfate, or demineralized bone matrix.Alternatively, the porous coating may carry osteoinductive agents, suchas bone morphogenic proteins, including rhBMP-2 and rhBMP-7.

Another embodiment of the present invention provides a flange extendingfrom the prosthetic facet. The flange is oriented relative to the bodyof the prosthesis such that when the flange is placed against thepedicle and in a manner such that the planar surface of the flange isperpendicular to the axis of the pedicle interior bone canal, thearticulating surface of the prosthesis will be properly positioned tomatch the articulating surface of the natural facet. The flange includesa hole for the passage of a fastener to securely attach the prosthesisto the pedicle. The fastener can be a screw, spike, tack, staple, or thelike.

Because the present invention allows for the individual replacements offacets, only compromised facets need be replaced. For example, if onlyone facet is affected by disease or trauma, it can be resected andreplaced with a facet prosthesis that articulates with an opposingnatural facet.

The present invention has numerous advantages over the prior art. Oneadvantage is that the quality of attachment of the prosthesis isimproved. The present invention provides a precise and tight press fitinto bones, as opposed to relying on prosthetic surfaces mating withhighly complex and variable external surfaces of the vertebra, such asthe posterior arch or facet. Another advantage is that the optionalporous coating is placed into interior bone spaces where porous coatingshave proven to achieve bone ingrowth for excellent long term fixationstrength. This ability to achieve bone ingrowth is uncertain for theprior art devices that engage the external bone surfaces of thevertebra. Yet another advantage lies in the removal of the facet bonestructure; where the facet bone is involved in the disease pathology orthe trauma that compromised the articular or cartilaginous surface ofthe facet, resection provides a means for ensuring that all painassociated with the disease or trauma is removed and the true joint lineis restored. With prior art devices, the bony structure of the facetswas generally left intact.

The above, and other objects, features and advantages of the presentinvention, will become apparent from the following description which isto be read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will bemore fully disclosed or rendered obvious by the following detaileddescription of the preferred embodiments of the invention, which is tobe considered together with the accompanying drawings wherein likenumbers refer to like parts and further wherein:

FIG. 1 is a perspective view of a portion of the spine;

FIG. 1A is a dorsal view of the portion of the spine shown in FIG. 1;

FIG. 2 is a lateral view of a facet joint reconstructed in accordancewith the present invention;

FIG. 3 is a dorsal view of the facet joint shown in FIG. 2;

FIG. 4 is a perspective view of the implanted left inferior facetprosthesis shown in FIGS. 2 and 3;

FIG. 5 is a perspective view of the left inferior facet prosthesis shownin FIGS. 2 and 3;

FIG. 6 is a cranial view of the implanted left superior facet prosthesisshown in FIGS. 2 and 3;

FIG. 7 is a perspective view of the left superior facet prosthesis shownin FIGS. 2 and 3;

FIG. 8 is a perspective view of an alternate implanted left superiorfacet prosthesis;

FIG. 9 is a perspective view of an alternate left superior facetprosthesis;

FIG. 10 is a lateral view of an alternative reconstructed facet joint;

FIG. 11 is a dorsal view of an alternative reconstructed facet joint;

FIG. 12 is a perspective view of the implanted left inferior facetprosthesis shown in FIGS. 10 and 11;

FIG. 13 is a perspective view of the alternative left inferior facetprosthesis shown in FIGS. 10 and 11;

FIG. 14 is a cranial view of the alternative implanted left superiorfacet prosthesis FIGS. 10 and 11

FIG. 15 is a perspective view of the alternative left superior facetprosthesis shown in FIGS. 10 and 11; and

FIG. 16 is a perspective view of an alternate bearing surface for thesuperior facet is shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 1A, there is shown a superior vertebra 1and an inferior vertebra 3, with an intervertebral disc 2 located inbetween. Vertebra 1 has superior facets 43, inferior facets 6, posteriorarch 35 and spinous process 46. Vertebra 3 has superior facets 7,inferior facets 44, posterior arch 36 and spinous process 45.

Referring now to FIG. 2, the left inferior facet 6 of vertebra 1 hasbeen resected and an inferior facet prosthesis 4 has been attached tovertebra 1. Similarly, the left superior facet of vertebra 3 has beenresected and a superior facet prosthesis 5 has been attached to vertebra3.

FIG. 3 illustrates a dorsal view of the elements shown in 2. It can beappreciated that inferior facet prosthesis 4 replicates the naturalanatomy when compared to the contralateral inferior facet 6 of vertebra1. Similarly, it can be appreciated that superior facet prosthesis 5replicates the natural anatomy when compared to the contralateralsuperior facet 7 of vertebra 3.

Turning now to FIG. 4, a perspective view of vertebra 1 with implantedinferior facet prosthesis 4 is provided. Resection at 31 has removed thenatural inferior facet 6 at the bony junction between the inferior facet6 and the posterior arch 35. In this manner, any bone pain associatedwith a disease, such as osteoarthritis, or trauma will be eliminated asthe involved bony tissue has been osteotomized

FIG. 5 illustrates a perspective view of inferior facet prosthesis 4.Surface 8 replicates the natural articular surface of the replacedinferior facet 6. Post 9 provides a means to affix inferior facetprosthesis 4 to vertebra 1. Post 9 is implanted into the interior bonespace of the left pedicle P (FIG. 6) on vertebra 1 and may or may notextend into the vertebral body of vertebra 1 to provide additionalstability.

FIG. 6 illustrates a cranial view of vertebra 3 with implanted superiorfacet prosthesis 5. Resection surface 32 represents the bony junctionbetween the natural superior facet and the posterior arch 35.

FIG. 7 illustrates a perspective view of superior facet prosthesis 5.Surface 36 replicates the natural articular surface of the replacedsuperior facet 7. Post 37 provides a means for affixing superior facetprosthesis 5 to vertebra 3. Post 37 is implanted into the interior bonespace of the left pedicle P (FIG. 6) on vertebra 3 and may or may notextend into the vertebral body of vertebra 3 to provide additionalstability.

When the total facet joint is replaced, as shown in FIGS. 2 and 3, thensurface 8 (FIG. 5) articulates with surface 36 (FIG. 7) to recreate thenatural biomechanics of the spine motion segment made up of vertebra 1,vertebra 3, and intervertebral disc 2. The post 37 has an implantableportion 50 (i.e., the portion of the post 37 that is implanted in to theinterior bone space of the left pedicle P in FIG. 6). The implantableportion 50 has a longitudinal axis 52 extending along its length, and alargest cross section 54 perpendicular to the longitudinal axis 52. Aprojection 56 of the largest cross section 54 extends posteriorly alongthe longitudinal axis 52. As shown in FIG. 7, the entire surface 36 ispositioned medially of the projection 56. The surface 36 is alsodisplaced from the longitudinal axis 52 by an offset 58.

FIG. 8 illustrates an alternative inferior facet prosthesis 10 which isimplanted into the interior bone space of posterior arch 35. Theinterior bone space is accessed from the resection 31.

FIG. 9 shows details of alternative inferior facet prosthesis 10,including the fin 13 that extends into the interior bone space ofposterior arch 35. Surface 12 replicates the natural articular surfaceof the replaced facet.

If desired, a corresponding fin construction can be used to form aprosthetic superior facet.

The surfaces of post 9 (FIG. 5), post 37 (FIG. 7) and fin 13 (FIG. 9)may or may not include porous coatings to facilitate bone ingrowth toenhance the long term fixation of the implant. Furthermore, such porouscoatings may or may not include osteoinductive or osteoconductivesubstances to further enhance the bone remodeling into the porouscoating.

Referring now to FIG. 10, there is shown a lateral view of a superiorvertebra 14 and an inferior vertebra 16, with an intervertebral disc 15located in between is shown. The left inferior facet of vertebra 14 hasbeen resected and an inferior facet prosthesis 18 has been attached tovertebra 14 by means of a screw fastener 17, wherein the screw fasteneris implanted into an interior bone space of the pedicle of the vertebra.Similarly, the left superior facet of vertebra 16 has been resected anda superior facet prosthesis 19 has been attached to vertebra 16 by meansof a screw fastener 17, wherein the screw fastener is implanted into aninterior bone space of the pedicle of the vertebra.

FIG. 11 illustrates a dorsal view of the elements of FIG. 10. It can beappreciated that inferior facet prosthesis 18 replicates the naturalanatomy when compared to the contralateral inferior facet 22 of vertebra14. Similarly, it can be appreciated that superior facet prosthesis 19replicates the natural anatomy when compared to the contralateralsuperior facet 21 of vertebra 16. In a preferred embodiment of thepresent invention, inferior facet prosthesis 18 and/or superior facetprosthesis 19 has a bone contacting surface which is porous coated toallow for bone ingrowth.

Turning now to FIG. 12, there is provided a perspective view of vertebra14 with implanted inferior facet prosthesis 18. Resection 34 has removedthe natural inferior facet at the bony junction between the inferiorfacet and the posterior arch 37. In this manner, any bone painassociated with a disease, such as osteoarthritis, or trauma will beeliminated inasmuch as the involved bony tissue has been osteotomized.

FIG. 13 illustrates a perspective view of inferior facet prosthesis 18.Surface 23 replicates the natural articular surface of the replacedfacet. Flange 25 contacts the pedicle and hole 24 receives a fastener toattach inferior facet prosthesis 18 to vertebra 14.

FIG. 14 illustrates a cranial view of vertebra 16 with implantedsuperior facet prosthesis 19. Resection surface 35 represents the bonyjunction between the natural superior facet and the posterior arch 38.

FIG. 15 illustrates a perspective view of superior facet prosthesis 19.Surface 27 replicates the natural articular surface of the replacedfacet. Flange 39 contacts the pedicle and hole 26 receives a fastener toattach inferior facet prosthesis 19 to vertebra 16.

FIG. 16 illustrates an alternative superior facet prosthesis 40 with anbearing surface 41 that mounts to substrate 42. The bearing surface 41is a biocompatible polymeric material, such as ultra high molecularweight polyethylene. Alternately, the bearing surface can be ceramic,such as zirconia or alumina, or metal. The substrate is a biocompatiblemetal alloy, such as an alloy of titanium, cobalt, or iron.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that theembodiments shown herein are by way of example, and that various changesand modifications may be effected by one skilled in the art withoutdeparting from the scope or spirit of the invention as defined in thefollowing claims.

1. A system for replacing at least a portion of a natural facet of afirst vertebra, the system comprising: a first prosthesis comprising: afixation element implantable in the first vertebra, the fixation elementhaving a longitudinal axis, the fixation element comprising animplantable portion having a largest cross section perpendicular to thelongitudinal axis; and a first superior articular surface shaped toarticulate with a first inferior articular surface on a second vertebra;wherein the first superior articular surface is connectable to thefixation element such that the entire first superior articular surfaceis positioned medially of a projection of the largest cross sectionextending posteriorly along the longitudinal axis; wherein the firstsuperior articular surface faces medially; wherein the first superiorarticular surface is offset from the longitudinal axis; wherein thefirst superior articular surface is not formed as a single piece withany inferior articular surface.
 2. The system of claim 1, wherein atleast part of the first superior articular surface is also displacedposteriorly from at least part of the projection.
 3. The system of claim1, wherein the first prosthesis is configured to be mountable to thefirst vertebra without being supported by a lamina of the firstvertebra.
 4. The system of claim 1, wherein the first vertebra comprisesa lumbar vertebra.
 5. The system of claim 1, wherein the firstprosthesis is configured to replace a resected portion of a firstsuperior articular process of the first vertebra.
 6. The system of claim1, further comprising a second prosthesis comprising a second superiorarticular surface shaped to articulate with a second inferior articularsurface on the second vertebra.
 7. The system of claim 6, furthercomprising: a third prosthesis mountable to the second vertebra, thethird prosthesis comprising the first inferior articular surface; and afourth prosthesis mountable to the second vertebra, the fourthprosthesis comprising the second inferior articular surface.
 8. A methodfor replacing at least a portion of a natural facet of a first vertebra,the method comprising: implanting an implantable portion of a fixationelement in the first vertebra, the fixation element having alongitudinal axis, the fixation element comprising an implantableportion having a largest cross section perpendicular to the longitudinalaxis; and positioning a first superior articular surface of a firstprosthesis to articulate with a first inferior articular surface on asecond vertebra, such that the entire first superior articular surfaceis positioned medially of a projection of the largest cross sectionextending posteriorly along the longitudinal axis; wherein the firstsuperior articular surface faces medially; wherein the first superiorarticular surface is offset from the longitudinal axis; wherein thefirst superior articular surface is not formed as a single piece withany inferior articular surface.
 9. The method of claim 8, whereinpositioning the first superior articular surface further comprisespositioning at least part of the first superior articular surfaceposteriorly from at least part of the projection.
 10. The method ofclaim 8, further comprising mounting the first prosthesis to the firstvertebra with the fixation element without supporting the firstprosthesis with a lamina of the first vertebra.
 11. The method of claim8, wherein mounting the first prosthesis to the first vertebra comprisesmounting the first prosthesis to a lumbar vertebra.
 12. The method ofclaim 8, further comprising: resecting a portion of a first superiorarticular process of the first vertebra; and mounting the firstprosthesis to the first vertebra to replace the resected portion of thefirst superior articular process.
 13. The method of claim 8, furthercomprising positioning a second superior articular surface of a secondprosthesis to articulate with a second inferior articular surface on thesecond vertebra.
 14. The method of claim 13, further comprising:mounting a third prosthesis to the second vertebra, the third prosthesiscomprising the first inferior articular surface; and mounting a fourthprosthesis to the second vertebra, the fourth prosthesis comprising thesecond inferior articular surface.